In many cases it is useful to know if a gaseous compound is present in a sample and to determine its concentration. The medical field is the most important sector where such information can be very useful. In fact a blood gas analysis is performed on many hospital patients both during and after surgery. Three parameters of interest in the blood analysis are the partial pressures of oxygen (pO.sub.2) and carbon dioxide (pCO.sub.2) and the negative logarithm of hydrogen ion activity (the pH).
Other fields in which this kind of information can be applied are industrial processes wherein a gas container is involved: for example, an oil containing tank, a container wherein some chemical reactions occur, a fermentation process for the production of food, and the like.
Additional fields of application also can be considered. For example, it could be useful to know about the oxygen or carbon dioxide gas presence when mining in a tunnel, or when operating in any underground areas. This advances the safety of people who have to work for many hours in such environmental conditions.
In the medical field, luminescent aromatic molecules have been found useful to detect the oxygen presence or the carbon dioxide presence in blood samples. These molecules are not in direct contact with the liquid medium, i.e. blood, but they are separated by a polymeric membrane permeable to oxygen molecules and not permeable to water molecules. In such a way the pO.sub.2 and the pCO.sub.2 at the luminescent molecules interface are, respectively, proportional to the concentration of the oxygen and of carbon dioxide in blood. For that reason, the suitability of a sensor to detect pO.sub.2 or pCO.sub.2 changes can be assessed separately, in a gaseous atmosphere simulating that in equilibrium with blood.
Generally the presence of carbon dioxide in a sample, for example, blood, is performed by fiber optic fluorescent sensors having a pH sensitive dye which detects changes in hydrogen ion concentration in a bicarbonate containing solution, which in turn varies as a function of the level of carbon dioxide present in the blood according to the mass action law (see Gehrich et al., IEEE Trans. on Biomedical Engineering, Vol. BME-33, No. 2, pp. 117-132, February 1986). By this way, the presence of carbon dioxide is not measured in a direct way, but as a consequence of the level of pH present in the sample. These kinds of measuring systems can determine some mistakes and sometimes the data obtained are not very precise. It could be useful to have a method which directly supply the carbon dioxide values in the sample, without measuring the pH level.
Additional methods to monitor carbon dioxide concentration in a sample are performed by using solid electrochemical cells, or by using calorimetric cells based on calcium ion-exchanged A-type zeolite and on carbonation of hydroxyapatite, respectively, or by using electrical resistence change associated to the electrochemical reduction of CO.sub.2, or by using oxide capacitors consisting of mixtures of BaTiO.sub.3 with oxides such as PbO, CuO, NiO (see Ishihara et al., Journal of Electroch. Soc., Vol. 138-1, 173, 1991).
All such methods to monitor carbon dioxide are not always satisfactory with respect to sensitivity and long term stability. Such methods may often require high working temperature conditions.